At present, one of the methods by which a sample in a specimen may be quantified with the highest sensitivity and accuracy is mass spectrometry. What is important for the measurement of a liquid sample by mass spectrometry is to select an ionization method suited for the characteristics of the compound. For example, when classified based on the factors by which the compound is made unstable, the ionization methods are largely classified into hard ionization and soft ionization. The former includes electron ionization method, chemical ionization method and atmospheric pressure ionization method, and the latter includes electrospray ionization method, matrix-assisted laser desorption ionization method and fast atom bombardment ionization.
On the other hand, a method (LC/MS) in which the sample separated by high performance liquid chromatography is subjected to mass spectrometry on the same the ionization method suited for the liquid chromatography is a method for directly ionizing the sample solution flowing out from the tip of a capillary, the ionization method is classified into the latter method in the methods described above. In particular, the apparatus in which an electrospray ionization mass spectrometer and an apparatus for high performance liquid chromatography are connected on line is most widely used, and well exhibits its power in identification of substances indispensable to the body, such as proteins and saccharides, as well as of environmental hormones. In the electrospray ionization, the possibility that the molecular ion peak of the sample is detected is extremely low, and the sample is detected as a pseudo-molecular ion peak in which sodium is usually attached.
Alternatively, in LC/MS, ionization of the sample is aided by admixing a protic solvent such as ammonium acetate, formic acid or acetic acid to the mobile phase.
As a method for aiding ionization in the mass spectrometry of a liquid sample, it is now common to add a protic solvent to the mobile phase of liquid chromatography in LC/MS. However, the following drawbacks have been pointed out: i) when using ammonium acetate in anion mode, the ion of the sample and the ammonium ion are paired, so that the sensitivity is decreased; ii) when using trifluoroacetic acid in cation mode, the sample ion and the trifluoroacetic acid ion are paired, so that the sensitivity is decreased, and in anion mode, ionization is prevented except for some cases; and iii) in case of using acetonitrile, an acid must be added, and in this case, ammonium acetate cannot be used because it is not dissolved therein.